I'd do a LITTLE reverse engineering. It looks like a 5V supply in the HDMI thingy. So what's the 3.3 V for
Parallel LEDs get funky. I have done it before. Parallel LED's work reasonably well when the LED voltages are the same with a reduction in intensity.
At least take a pic of the circuit board,
You also need to measure the voltage across the LED, both on and off.
And see what you can discover about the driver.
The output of the OPTO will always show high when presented to the analog input.
You should be able to use a digital input with a pull up resistor. e.g. +3.3..resistor..C..E..GND
An OPTO-FET might be a better choice.
I've been contemplating a more robust LED interface for other projects. It just hasn't got off off the ground. Well, maybe a little